Currently, a non-aqueous electrolyte secondary battery including a lithium ion secondary battery, which is used for a mobile device such as a mobile phone, is available as a commercial product. The non-aqueous electrolyte secondary battery generally has a constitution that a positive electrode having a positive electrode active substance or the like coated on a current collector and a negative electrode having a negative electrode active substance or the like coated on a current collector are connected to each other via an electrolyte layer in which a non-aqueous electrolyte solution or a non-aqueous electrolyte gel is maintained within a separator. According to absorption and desorption of ions such as lithium ions on an electrode active substance, charging and discharging reactions of a battery occur.
In recent years, it is desired to reduce the amount of carbon dioxide in order to cope with the global warming. As such, a non-aqueous electrolyte secondary battery having small environmental burden has been used not only for a mobile device or the like but also for a power source device of an electric vehicle such as a hybrid vehicle (HEV), an electric vehicle (EV), and a fuel cell vehicle.
As the non-aqueous electrolyte secondary battery for application to an electric vehicle, it is required to have high output and high capacity. As a positive electrode active substance used for the positive electrode of a non-aqueous electrolyte secondary battery for an electric vehicle, a lithium cobalt composite oxide, which is a layered composite oxide, has been already widely used since it can provide high voltage at the level of 4 V and has high energy density. However, due to resource scarcity, cobalt as a raw material is expensive, and considering the possibility of having dramatic demand in future, it is not stable in terms of supply of a raw material. There is also a possibility of having an increase in the raw material cost of cobalt. Accordingly, a composite oxide having less cobalt content ratio is desired.
Similarly to a lithium cobalt composite oxide, a lithium nickel composite oxide has a layered structure. In addition, it is less expensive than the lithium cobalt composite oxide and is almost equivalent to the lithium cobalt composite oxide in terms of theoretical discharge capacity. From this point of view, it is expected that a lithium nickel composite oxide (for example, LiNiO2 or Li(Nix, Coy, Mnz)O2 (x+y+z=1, x>y, x>z) or the like) is used for constituting a battery with high capacity for practical use.
According to a lithium ion secondary battery in which a lithium nickel composite oxide is used for a positive electrode active substance, charging and discharging occurs as a result of desorption and insertion of lithium ions to the nickel composite oxide. At that time, the composite oxide undergoes shrinkage and expansion in conjunction with the desorption and insertion of lithium ions. For such reasons, although stable insertion and desorption of lithium ions occurs on a surface of an active substance particle, it is difficult to have insertion and desorption of lithium ions in a center part so that deviation in Li concentration may easily occur within an active substance particle. In addition, a crack occurs in the particle due to a factor such as the collapse of the crystal structure or the like. According to this crack, the conductive path is lost, which causes a decrease in capacity or an increase in resistance of a battery. There is also a problem such as poor durability, that is, a huge decrease in capacity is caused according to repeated charge and discharge cycle and the decrease in capacity becomes significant when the battery is used for a long period of time.
In view of the aforementioned problems, Patent Document 1, for example, is characterized in that having relatively large primary particles for forming secondary particles in a lithium nickel composite oxide is suggested to lower the porosity of the secondary particles. According to the technique described in Patent Document 1, relatively large primary particles are used to lower the porosity of the secondary particles, that is, it is suggested to suppress a decrease in capacity by increasing the density of a positive electrode active substance.